The nuclear lamina is closely apposed to the inner membrane of the nuclear envelope (NE) and contributes to the size, shape and mechanical stability of the nucleus. The major structural proteins of the lamina are the A- and B-type nuclear lamins, comprised of lamins A and C (LA, LC), and lamins B1 (LB1) and B2 (LB2). Although the major fraction of lamins is found in the lamina, they are also located throughout the nucleoplasm (Moir et al. 2000b; Shimi et al. 2008; herein incorporated by reference in their entireties). LA and LC are derived from a single gene (LMNA) by alternative splicing and are expressed only in differentiated cells. LB1 and LB2 are encoded by LMNB1 and LMNB2, respectively, and at least one of the B-type lamins is expressed in all cells throughout development (Dechat et al. 2008; herein incorporated by reference in its entirety). There is growing evidence that the nuclear lamins play important roles in the anchorage of peripheral elements of chromatin, in regulating the organization of chromosome territories and in gene expression (Kosak et al. 2002; Guelen et al. 2008; Kumaran and Spector 2008; Shimi et al. 2008; herein incorporated by reference in their entireties). The lamins have also been shown to play important roles in DNA replication and repair, RNApolymerase II transcription, and the epigenetic control of chromatin remodeling (Moir et al. 2000a; Spann et al. 2002; Shimi et al. 2008; Shumaker et al. 2008; herein incorporated by reference in their entireties).
All lamins share a common structure with a conserved α-helical central rod domain flanked by globular head and tail domains (Dechat et al. 2008; herein incorporated by reference in its entirety). The central rod domains of two lamins dimerize into in-parallel and in-register coiled-coil structures which then interact head-to-tail to form long protofilaments. Lateral interactions between anti-parallel protofilaments, with influence from the head and tail domains, form the higher order structures found in the nucleus (Kapinos et al. 2010; herein incorporated by reference in its entirety). Using electron microscopy, the lamina in Xenopus oocytes appears as a meshwork of ˜10 to 15 nm filaments (Aebi et al. 1986; herein incorporated by reference in its entirety). Higher order LB1 structures organized into meshworks have been seen in mouse cells by immunofluorescence (Schermelleh et al. 2008; herein incorporated by reference in its entirety). Additionally, A- and B-type lamin fibrils form interacting meshworks within the lamina in HeLa cells (Shimi et al. 2008; herein incorporated by reference in its entirety). Support for these interactions has been derived from silencing LB1 expression using shRNA. The loss of LB1 leads to a dramatic increase in the size of the LA/C meshwork and induces the formation of LA/C-rich and LB2-deficient NE blebs. These findings demonstrate that LB1 plays an essential role in maintaining normal LA/C and LB2 meshwork structures (Shimi et al. 2008; herein incorporated by reference in its entirety). Furthermore, the LA/C-rich NE blebs induced by LB1 silencing contain gene rich chromatin with low transcriptional activity even though the activated form of RNA polymerase II (Pol IIo) is enriched in these regions. This suggests that both A- and B-type lamins are required for properly regulating gene expression (Shimi et al. 2008). Interestingly, the nuclei of cells from patients with diseases caused by mutations in LMNA, such as Hutchinson-Gilford Progeria and Emery-Dreifuss Muscular Dystrophy type 2, frequently exhibit alterations in the structural organization of the A- and B-type lamin meshworks and contain NE blebs similar to those induced by silencing LB1 expression (Goldman et al. 2004; Meaburn et al. 2007; Shimi et al. 2008; Taimen et al. 2009; herein incorporated by reference in their entireties).
The detection of senescent cells is an important parameter for all types of biomedical research studies as well as in pathology, aging and cancer.